Twisted liquid-crystal materials generally have smectic, nematic or cholesteric phases. Liquid-crystal mixtures frequently contain one or more optically active components in order to induce a chiral structure. For example, cholesteric liquid crystals can comprise a nematic base material and one or more optically active dopes, which produce either a right-handed or left-handed twist in the nematic material. Cholesteric liquid crystals reflect light in a wavelength range for which the wavelength is approximately equal to the helix pitch (.lambda.=n.multidot.p). The reflected light is fully circular-polarized. The direction of rotation of the reflected light depends on the directional rotation of the cholesteric helix structure. The light circular-polarized in the opposite direction in each case is transmitted at the same strength.
A large number of optically active dopes which are more or less suitable for certain purposes are described in the literature. U.S. Pat. No. 4,996,330 discloses chiral N-substituted tartarimides which are diesterified at the hydroxyl groups by mesogenic radicals. Such substances can be used as additives in electro-optical display devices, such as TN or STN cells, in order to achieve the requisite twist.
Polymeric or crosslinked cholesteric liquid crystals enable more extensive applications. They can be used to prepare LC pigments having novel effects or reflective polarizing filters, inter alia. The incorporation of methacrylates of cholesterol or derivatives thereof as dopes into liquid-crystalline polymers is described by H. Finkelmann, H. Ringsdorf et al., in Makromol. Chem. 179, 829-832 (1978).
EP-A-358 208 describes cyclic siloxanes containing mesogenic side groups, in which some of the side groups have been esterified by methacrylic acid. These siloxanes can therefore also be crosslinked. The dopes incorporated into these polymers are hydrosilylated allyloxybenzoates of cholesterol or derivatives thereof. However, they have only a relatively low helical twisting power (abbreviated to HTP) and must therefore be added in large amounts (about 50%) in order to cause color effects in the visible region. The large amount of dopes means that the crosslinker group density cannot be chosen to be very high. In addition, the chiral dopes are relatively expensive.
For the construction of some optical filters, a large spectral width of the reflection light is of crucial importance. The spectral width of the reflection light can be increased by a high proportion of aromatic compounds in the liquid crystal. However, cholesterol-containing systems have a high content of cyclohexyl rings. The spectral width of the reflection light is thus frequently too small for these applications.
For some applications of optical filters, it would furthermore be desirable to reflect circular-polarized light of both directions of rotation. The above mentioned cholesterol-containing systems generally likewise do not offer this possibility.